The present invention has utility especially in the field of modulating valves which operate with pressure differences of no greater than about two atmospheres. It is described herein as a portion of a solenoid control valve which issues an output control pressure produced as an intermediate pressure between a substantially constant higher pressure and a substantially constant lower pressure supplied to the solenoid valve. The solenoid and spring arrangements of the valve effect output pressures corresponding quantitatively to the level of electrical activation of the solenoid. Such a valve when energized by an electrical source and a vacuum source on a tow vehicle to operate a vacuum booster on a trailer, provides an output control pressure developed from atmosphere as the higher pressure, and engine vacuum as the lower pressure.
Such a valve is typically arranged so that at normal inoperative conditions, the output control pressure assumes the level of one of the two supply pressures, e.g., in the valve to be described, the engine vacuum pressure by way of an intervening vacuum reservoir. Obviously, modulation of the control pressure is thus effected by leaking air or other fluid from the higher pressure source into control pressure region of the valve while shutting or substantially reducing connection of the control pressure region with the vacuum source.
The main problem dealt with by this invention is that of providing valve mechanism within a modulating valve for releasing the higher pressure fluid into the lower-pressured control pressure region with the least possible "hysteresis" in the valve. Hysteresis is herein regarded as effort expended within the valve to overcome lag in operation due to friction of relatively moving parts. Hysteresis is particularly to be avoided in control valves at low level control efforts.
Conventional O-ring seal configurations typically constrain the seal ring by tight entrapment to obtain the compression of the O-ring necessary for sealing mating surfaces. Such construction is accompanied by friction sought to be eliminated by this invention. Moreover, a ratio of normal manufacturing tolerances of valve parts to O-ring cross section in the conventional configuration results in a wide highly undesirable variation in seal ring compression and in the friction of the ring with the cooperating sealing valve portion. Such frictional variation is especially significant in the low level control efforts of modulating valves and in the manufacturing of valves to uniform performance.